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Abstract:

An implantable medical device includes electrodes that are configured to
be positioned within at least one of a heart and a chest wall of a
patient. The device also includes an impedance measurement module, a
patient position sensor, and a correction module. The impedance
measurement module measures an impedance vector between a predetermined
combination of the electrodes. The patient position sensor determines at
least one of a posture and an activity level of the patient. The
correction module adjusts the impedance vector based on the at least one
of the posture and the activity level of the patient.

Claims:

1. An implantable medical device comprising: electrodes configured to be
positioned within at least one of a heart and chest wall of a patient; an
impedance measurement module to measure an impedance value between a
predetermined combination of the electrodes; a patient position sensor to
determine at least one of a posture and an activity level of the patient;
and a correction module to adjust the impedance value based on the at
least one of the posture and the activity level of the patient.

2. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance value by applying an offset factor to the
impedance value, the offset factor having a value that varies based on
the at least one of the posture and the activity level of the patient.

3. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance values by applying an offset factor to the
impedance value, the offset factor based on a comparison between acute
and chronic changes in previously obtained impedance values following a
change in the posture of the patient.

4. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance values by applying an offset factor to the
impedance value, the offset factor based on chronic changes in previously
obtained impedance values following a change in the posture of the
patient.

5. The implantable medical device of claim 4, wherein the chronic changes
in the previously obtained impedance values include a difference between
the previously obtained impedance values that were measured at least one
hour after the change in the posture of the patient.

6. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance values by an offset factor, the offset
factor based on acute changes in previously obtained impedance values
following a change in the posture of the patient.

7. The implantable medical device of claim 6, wherein the acute changes
in previously obtained impedance values include a difference between the
previously obtained impedance values that were measured within one minute
after the change in the posture of the patient.

8. The implantable medical device of claim 1, wherein the posture is a
current posture and the correction module continues to adjust impedance
values measured by the impedance measurement module between the
predetermined combination of electrodes by applying an offset factor to
the impedance measurements for a predetermined time period after the
patient changes from a previous posture to the current posture.

9. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance value by selecting an offset factor from a
plurality of offset factors and applying the offset factor to the
impedance value, the offset factor selected from the plurality of offset
factors based on the predetermined combination of electrodes used to
measure the impedance value.

10. The implantable medical device of claim 1, wherein the correction
module adjusts the impedance value by selecting an offset factor from a
plurality of offset factors and applying the offset factor to the
impedance value, the offset factor selected from the plurality of offset
factors based on the at least one of the posture and the activity level
of the patient.

11. The implantable medical device of claim 1, wherein the correction
module uses the at least one of the posture and the activity level of the
patient to adjust a left atrial pressure estimate of the patient.

12. A method for adjusting an impedance value obtained by a medical
device, the method comprising: measuring the impedance value using a
predetermined combination of electrodes that are positioned in at least
one of a heart and a chest wall of a patient; determining at least one of
a posture and an activity level of the patient when the impedance value
is measured; and adjusting the impedance value based on the at least one
of the posture and the activity level of the patient.

13. The method of claim 12, wherein the adjusting operation comprises
applying an offset factor to the impedance value, the offset factor
having a value that varies based on the at least one of the posture and
the activity level of the patient.

14. The method of claim 12, wherein the adjusting operation comprises
applying an offset factor to the impedance value, the offset factor based
on a comparison between acute and chronic changes in previous obtained
impedance values following a change in the posture of the patient.

15. The method of claim 12, wherein the adjusting operation comprises
applying an offset factor to the impedance value, the offset factor based
on chronic changes in previously obtained impedance values following a
change in the posture of the patient.

16. The method of claim 12, wherein the adjusting operation comprises
applying an offset factor to the impedance value, the offset factor based
on acute changes in previously obtained impedance values following a
change in the posture of the patient.

17. The method of claim 12, wherein the posture is a current posture and
the adjusting operation continues to adjust impedance values measured
between the predetermined combination of electrodes by applying an offset
factor to the impedance measurements for a predetermined time period
after the patient changes from a previous posture to the current posture.

18. The method of claim 12, wherein the adjusting operation comprises
selecting an offset factor from a plurality of offset factors and
applying the offset factor to the impedance value, the offset factor
selected from the plurality of offset factors based on the predetermined
combination of electrodes used to measure the impedance value.

19. The method of claim 12, wherein the adjusting operation comprises
selecting an offset factor from a plurality of offset factors and
applying the offset factor to the impedance value, the offset factor
selected from the plurality of offset factors based on the at least one
of the posture and the activity level of the patient.

20. A system comprising: means for measuring an impedance value using a
predetermined combination of electrodes that are positioned in at least
one of a heart and a chest wall of a patient; means for determining at
least one of a posture and an activity level of the patient; and means
for adjusting the impedance value based on the means for determining.

Description:

FIELD OF THE INVENTION

[0001] Embodiments described herein generally pertain to implantable
medical devices and more particularly to methods and devices that obtain
impedance vectors between electrodes positioned within a heart and/or
chest wall.

BACKGROUND OF THE INVENTION

[0002] An implantable medical device (IMD) is implanted in a patient to
monitor, among other things, electrical activity of a heart and to
deliver appropriate electrical therapy, as required. IMDs include
pacemakers, cardioverters, defibrillators, implantable cardioverter
defibrillators (ICD), and the like. The electrical therapy produced by an
IMD may include pacing pulses, cardioverting pulses, and/or defibrillator
pulses to reverse arrhythmias (for example, tachycardias and
bradycardias) or to stimulate the contraction of cardiac tissue (for
example, cardiac pacing) to return the heart to its normal sinus rhythm.
These pulses are referred to as stimulus or stimulation pulses.

[0003] IMDs may monitor electrical characteristics of the heart to
identify or classify cardiac behavior and to estimate physiological
parameters of the heart. For example, some known IMDs measure
intracardiac and intrathoracic impedance vectors between combinations of
electrodes in the heart and/or chest wall to estimate left atrial
pressure (LAP) in the heart. As the left atrium of the heart fills with
fluid and the LAP increases, the impedance measured between two
electrodes and along a vector that traverses the left atrium may
decrease. Conversely, as the fluid level in the left atrium drops, the
LAP may decrease and the impedance vector through the left atrium may
increase.

[0004] In order to use intracardiac and intrathoracic impedance vectors to
estimate LAP, the IMD may need to be calibrated so that a measured
impedance vector may be accurately transformed into a corresponding
estimate of LAP. Additionally, the IMD may be unable to compensate for
changes in the posture of the patient because such changes can produce
changes in the interelectrode spacing and geometry that may impact the
measured impedance. For example, when a patient changes posture from a
supine to an upright standing position an acute change in the
interelectrode spacing may occur in combination with the expected
decrease in the intracardiac and intrathoracic fluid volume associated
with this posture maneuver. The acute change in interelectrode spacing
may cause the measured impedance to either increase or decrease or not
change at all. The acute decrease in intracardiac and intrathoracic fluid
volume will cause the measured impedance to increase since impedance is
inversely proportional to fluid volume. The overall effect of the acute
change in interelectrode spacing and intracardiac/intrathoracic fluid
volumes may cause the impedance measurement to either acutely increase or
decrease depending on the relative magnitude and direction of the change
associated with the change in interelectrode spacing. In either
situation, the impedance vectors may provide an unreliable indicator of
the LAP if the algorithm utilized to transform the measured impedance
into an estimate of LAP did not compensate for changes in impedance that
are a consequence of posture dependent rather than fluid volume dependent
changes in interelectrode spacing and geometry.

[0005] A need exists for a device and method for adjusting impedance
vectors or measurements to account for changes in interelectrode spacing
and geometry that occur after a patient changes positions or postures.

SUMMARY

[0006] In one embodiment, an implantable medical device is provided. The
implantable medical device includes electrodes that are configured to be
positioned within at least one of a heart and a chest wall of a patient.
The device also includes an impedance measurement module, a patient
position sensor, and a correction module. The impedance measurement
module measures an impedance value (or vector) between a predetermined
combination of the electrodes. The patient position sensor determines at
least one of a posture and an activity level of the patient. The
correction module adjusts the impedance value (or vector) based on the at
least one of the posture and the activity level of the patient.

[0007] In another embodiment, a method for adjusting an impedance value
(or vector) obtained by a medical device is provided. The method includes
measuring the impedance value using a predetermined combination of
electrodes that are positioned in at least one of a heart and a chest
wall of a patient and determining at least one of a posture and an
activity level of the patient when the impedance value is measured. The
method also includes adjusting the impedance value based on the at least
one of the posture and the activity level of the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The drawings illustrate generally, by way of example, but not by
way of limitation, various embodiments discussed in the present document.

[0009] FIG. 1 illustrates an IMD that is coupled to a heart of a patient
in accordance with one embodiment.

[0010] FIG. 2 is a schematic diagram of the IMD and the heart shown in
FIG. 1 when the patient is in a supine position in accordance with one
embodiment.

[0011] FIG. 3 is a schematic diagram of the IMD and the heart shown in
FIG. 1 when the patient is in an upright position.

[0012] FIG. 4 is a flowchart of a method for adjusting impedance vectors
based on changing postures of a patient in accordance with one
embodiment.

[0013] FIG. 5 illustrates a block diagram of exemplary internal components
of the IMD shown in FIG. 1 in accordance with one embodiment.

[0014] FIG. 6 illustrates a functional block diagram of an external
programming device shown in FIG. 5 in accordance with one embodiment.

[0015] FIG. 7 illustrates a distributed processing system in accordance
with one embodiment.

[0016] FIG. 8 illustrates a block diagram of exemplary manners in which
embodiments of the present invention may be stored, distributed and
installed on a tangible and non-transitory computer-readable medium.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In the following detailed description, reference is made to the
accompanying drawings which form a part hereof, and in which are shown by
way of illustration specific embodiments in which the present invention
may be practiced. These embodiments, which are also referred to herein as
"examples," are described in sufficient detail to enable those skilled in
the art to practice the invention. It is to be understood that the
embodiments may be combined or that other embodiments may be utilized,
and that structural, logical, and electrical variations may be made
without departing from the scope of the present invention. The following
detailed description is, therefore, not to be taken in a limiting sense,
and the scope of the present invention is defined by the appended claims
and their equivalents. In this document, the terms "a" or "an" are used,
as is common in patent documents, to include one or more than one. In
this document, the term "or" is used to refer to a nonexclusive or,
unless otherwise indicated. In this document the term "impedance vector"
refers to intracardiac and/or intrathoracic impedance measurements
derived from two or more electrodes positioned within the heart and/or
chest wall. In this document the term "admittance" is used to denote the
reciprocal of impedance.

[0018] In accordance with certain embodiments, methods and devices are
provided for adjusting impedance vectors obtained between predetermined
combinations of electrodes positioned within a heart and/or chest wall of
a patient. An impedance vector represents an impedance measurement
obtained along a path extending between the electrodes used to obtain the
impedance measurement. The impedance vectors are adjusted in order to
compensate for changes in the impedance measurements that are caused or
affected by posture dependent changes in the inter-electrode spacing
and/or geometry between the electrodes used to obtain the impedance
measurements. The changes in the inter-electrode spacing and/or geometry
between the electrodes may be caused by a shift or change in the posture
of the patient independent of changes in intracardiac and intrathoracic
fluid volume. The adjustments to the impedance measurements may prevent
the changing posture of the patient from causing inaccurate estimates of
various physiological parameters of the patient, such as left atrial
pressure (LAP) that is derived or based on the impedance measurements.

[0019] FIG. 1 illustrates an IMD 100 that is coupled to a heart 102 of a
patient in accordance with one embodiment. The IMD 100 may be a cardiac
pacemaker, an ICD, a defibrillator, an ICD coupled with a pacemaker, and
the like, implemented in accordance with one embodiment of the present
invention. The IMD 100 may be a dual-chamber stimulation device capable
of treating both fast and slow arrhythmias with stimulation therapy,
including cardioversion, defibrillation, and pacing stimulation, as well
as capable of detecting heart failure, evaluating its severity, tracking
the progression thereof, and controlling the delivery of therapy and
warnings in response thereto. As explained below in more detail, the IMD
100 may be controlled to obtain impedance or admittance vectors between
predetermined combinations of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 positioned within the heart 102 and adjust the
impedance or admittance vectors based on the posture of the patient.

[0020] The IMD 100 includes a housing 104 that is joined to receptacle
connectors 105, 106, 108 that are connected to a right ventricular (RV)
lead 110, a right atrial (RA) lead 112, and a coronary sinus lead 114,
respectively. The IMD 100 may be located in a patient's chest wall. The
leads 110, 112, 114 may be located at various locations, such as an
atrium, a ventricle, or both to measure physiological parameters of the
heart 102. One or more of the leads 110, 112, 114 detect IEGM signals
that form an electrical activity indicator of myocardial function over
multiple cardiac cycles. To sense atrial cardiac signals and to provide
right atrial chamber stimulation therapy, the RA lead 112 is joined with
an atrial tip electrode 116, which typically is implanted in the right
atrial appendage, and an atrial ring electrode 118. The coronary sinus
lead 114 receives atrial and ventricular cardiac signals and delivers
left ventricular pacing therapy using at least a left ventricular tip
electrode 120, delivers left atrial pacing therapy using at least a left
atrial ring electrode 122, and delivers shocking therapy using at least a
left atrial coil electrode 124. The coronary sinus lead 114 also includes
a left ventricular ring electrode 134 that is disposed between the LV tip
electrode 120 and the LV ring electrode 122. The RV lead 110 has right
ventricular tip electrode 126, a right ventricular ring electrode 128, a
right ventricular coil electrode 130, and an SVC coil electrode 132. The
RV lead 110 is capable of receiving cardiac signals, and delivering
stimulation in the form of pacing and shock therapy to the right
ventricle. The RV coil electrode 130 may be used as a defibrillation
electrode. For purposes of measuring impedance vectors between
predetermined combinations of the electrodes 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (as described below), the housing 104 of the IMD
100 may be referred to as an electrode.

[0021] In the illustrated embodiment, the IMD 100 includes a patient
position sensor 136. The patient position sensor 136 may be disposed
within the housing 104 or may be communicatively coupled with the IMD
100. The patient position sensor 136 is a device that determines a
position or orientation of the sensor 136. The sensor 136 may include a
multi-axis accelerometer that determines the orientation of the IMD 100.
As described below, the output of the sensor 136 may be used to determine
the posture or position of the patient along with an activity level. For
example, with respect to posture, the sensor 136 may be used to determine
if the patient is in one or more of the following positions: (i) upright,
or standing upright, (ii) supine, or laying on his or her back, (iii)
prone, or laying on his or her stomach, (iv) right side down, or laying
on his or her right side or arm, (v) left side down, or laying on his or
her left side or arm, or (vi) a combination of any of the previously
listed positions. A combination of positions that is detected by the
sensor 136 may be used to determine if the patient is laying between a
supine and right side down posture, or between a prone and a right side
down posture. The sensor 136 may be used to determine an activity level
of the patient by determining if the patient has recently switched or
changed postures or position and/or continues to switch or change
postures or positions.

[0022] The IMD 100 may measure one or more physiologic parameters of the
heart 102 in order to monitor a condition of the heart 102. For example,
the IMD 100 may obtain impedance or admittance vectors between
predetermined combinations of the electrodes 104, 116, 118, 120, 122,
124, 126, 128, 130, 132, 134 in order to monitor LA pressure (LAP) or
intracardiac pressures, ischemia of the heart 102, cardiac output, LA
wall velocity, cardiac heart failure indices, the beginning of pulmonary
edema, hemodynamic parameters, levels of fluid accumulation, and the
like.

[0023] An impedance vector is obtained by the IMD 100 between any two or
more of the electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132,
134. The impedance vector may be represented as the impedance measured
along a path (generally a linear path) between at least two points. One
or more impedance measurements obtained by the IMD 100 may extend through
the heart 102. The impedance vectors that extend through the heart 102
represent the impedance of the myocardium and the blood in the heart 102
along the paths of the impedance vectors. By way of example only, the IMD
100 may measure an impedance of the heart 102 along an impedance vector
138. As shown in FIG. 1, the impedance vector 138 extends between the LV
ring electrode 134 and the housing 104 of the IMD 100. Alternatively, the
IMD 100 may measure additional or different impedance vectors between any
two or more combinations of the electrodes 116, 118, 120, 122, 124, 126,
128, 130, 132, 134 and/or the housing 104. The impedance measured along
the impedance vector 138 may be expressed in terms of ohms.
Alternatively, the impedance may be expressed as an admittance
measurement. The admittance may be inversely related to the impedance. By
way of example only, the admittance along the impedance vector 138 may be
represented as:

A = 1000 Z ( Eqn . 1 ) ##EQU00001##

where "A" represents admittance in terms of 1/mΩ and "Z" represents
the impedance measurement in terms of ohms (Ω).

[0024] The impedance measured along the impedance vector 138 may vary
based on a variety of factors, including the amount of fluid in one or
more chambers of the heart 102 and/or thoracic space. As a result, the
impedance measurement may be indicative of LAP. As more blood fills the
left atrium and pulmonary veins, the LAP increases. Blood can be more
electrically conductive than air and/or the myocardium of the heart 102
along the impedance vector 138. Consequently, as the amount of blood in
the left atrium increases, the LAP increases and the impedance measured
along the impedance vector 138 may decrease. Conversely, decreasing LAP
may result in the impedance measurement increasing as there is less blood
in the left atrium and pulmonary veins.

[0025] But, inter-electrode spacing also may affect the impedance
measurements. For example, changes in posture of a patient from a supine
position, such as supine, prone, right side down, left side down, or a
combination thereof, to an upright standing position may result in
changes in the distance between the LV ring electrode 134 and the housing
104 of the IMD 100. Additionally, activity of a patient may vary the
distance between electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134. For example, movement of the patient may result in changes in
the distance between the LV ring electrode 134 and the housing 104.

[0026] FIG. 2 is a schematic diagram of the IMD 100 and the heart 102 when
the patient is in a supine position in accordance with one embodiment. As
shown in FIG. 2, an impedance vector 200 extends from an electrode 202 to
the IMD 100. The electrode 202 may be the LV ring electrode 134 (shown in
FIG. 1) such that the impedance vector 200 may extend from the LV ring
electrode 134 to a common point 204 on the housing 104 (shown in FIG. 1)
of the IMD 100. Alternatively, the electrode 202 may be a different
electrode 116, 118, 120, 122, 124, 126, 128, 130, 132 (shown in FIG. 1).
When the patient moves from the supine position represented in FIG. 2 to
another position or posture, the relative positions of the electrode 202
and the IMD 100 may change. Activity of the patient also may cause the
relative positions of the electrode 202 and IMD 100 to change.

[0027] FIG. 3 is a schematic diagram of the IMD 100 and the heart 102 when
the patient is in an upright standing position. As shown in FIG. 3, an
impedance vector 300 extends between the electrode 202 and the common
point 204 of the IMD 100. While both the impedance vectors 200, 300
extend between the electrode 202 and the common point 204 of the IMD 100,
the impedance vectors 200, 300 are oriented along different directions.
The impedance vectors 200, 300 are oriented along different directions
due to the change in posture of the patient. The changing posture from
supine posterior to upright causes the electrode 202 to move relative to
the IMD 100. This may occur as a consequence of the heart 102 dropping
down within the thoracic cavity when the patient stands upright, while
the IMD 100 that is attached to the chest wall remaining relatively
fixed. As a result, the impedance vector 200 shifts to the impedance
vector 300. If the impedance vectors 200, 300 do not extend over the same
distance and paths through the heart 102, the impedance measurements
obtained over the impedance vectors 200, 300 may differ.

[0028] In order to compensate for the change in the spacing or geometry
between the electrode 202 and the IMD 100 and the shift in the impedance
vector 200 to the vector 300, the IMD 100 may apply an offset factor
β to impedance measurements obtained along the impedance vector 200
or 300. The offset factor β is applied to impedance vectors 200, 300
in order to reduce or eliminate the impact of a changing posture of the
patient on the impedance vectors 200, 300. As the impact of posture on
the impedance vectors 200, 300 is reduced, the accuracy of physiologic
parameters such as LAP derived from the impedance vectors 200, 300 may be
increased. The offset factor β is derived based on impedance vectors
200, 300 measured between two electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132 (shown in FIG. 1) at different first and second
positions, such as a supine posture and an upright standing posture. The
offset factor β may then be applied to impedance vectors 200, 300
measured.

[0029] FIG. 4 is a flowchart of a method 400 for adjusting impedance
vectors based on changing postures of a patient in accordance with one
embodiment. The method 400 determines an offset factor β that can be
applied to impedance vectors that are measured between a predetermined
combination of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 (shown in FIG. 1) for a change in the patient's position from a
first posture to a second posture. The method 400 may be repeated several
times to determine additional offset factors β for different
combinations of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 and/or different changes in position.

[0030] At 402, a supine chronic admittance (AS) is measured between a
predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1) when the patient is in the
position of a first posture. The supine chronic admittance AS may be
obtained in a chronic ambulatory setting by measuring the impedance
vector between the predetermined combination of electrodes 104, 116, 118,
120, 122, 124, 126, 128, 130, 132, 134 after the patient has moved to the
first posture for a sufficiently long time period that fluids within the
patient's body have reached a steady state. For example, the supine
chronic admittance AS may be measured after a sufficient time to
allow the fluid in the various chambers of the heart 102 (shown in FIG.
1) and other thoracic chambers to reach a steady state after the patient
has moved to the first posture. In one embodiment, the first posture is a
supine position, but may also be a prone position, a right side down
position, or a left side down position.

[0031] The supine chronic admittance AS may be measured by measuring
the impedance vector between the predetermined combination of electrodes
104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 after the patient
have moved to the first posture, such as a supine position, and generally
remained in the first posture for at least four hours. Alternatively, the
supine chronic admittance AS may be obtained after the patient has
moved to the first posture for a different time period, such as thirty
minutes, one hour, two hours, five hours, and the like.

[0032] The supine chronic admittance AS may be measured as the
smallest impedance vector between the predetermined combination of
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 (shown
in FIG. 1) that is measured over a time window. The IMD 100 (shown in
FIG. 1) may periodically measure the impedance vector between the
predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 throughout the day and night. By way of example
only, the IMD 100 may measure the impedance vector every two hours
throughout the day and night. The IMD 100 may determine which of the
impedance vectors measured during the night (such as 10 p.m. to 6 a.m.)
is the smallest of the impedance vectors. The smallest impedance vector
obtained during the night may be obtained when the patient is likely to
be supine and corresponding to a period of time when intracardiac and
intrathoracic fluid volumes have reached a maximal state during the
night. The IMD 100 may then calculate the supine chronic admittance
AS from the impedance vector using Equation 1 above. In another
embodiment, the supine chronic admittance AS may be calculated based
on two or more impedance vectors and/or is based on an impedance vector
that is not the smallest impedance vector measured over a time window. By
way of example only, the supine chronic admittance AS may be one or
more of a mean, median, deviation, and the like, of several impedance
vectors obtained when the patient is likely to be supine.

[0033] At 404, an upright chronic admittance (AU) is measured between
the predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1) when the patient is in the
position of a second posture that differs from the first posture. The
upright chronic admittance AU may be obtained by measuring the
impedance vector between the predetermined combination of electrodes 104,
116, 118, 120, 122, 124, 126, 128, 130, 132, 134 after the patient has
moved to the second posture for a sufficiently long time period that
fluids within the patient's body have reached a steady state. For
example, the upright chronic admittance AU may be measured after a
sufficient time to allow the fluid in the various chambers of the heart
102 (shown in FIG. 1) and other thoracic chambers to reach a steady state
after the patient has moved to the second posture. In one embodiment, the
second posture is an upright standing position, such as when the patient
is vertically standing or sitting.

[0034] The upright chronic admittance AU may be obtained by measuring
the impedance vector between the predetermined combination of electrodes
104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 after the patient
have moved to the second posture and generally remained in the second
posture for at least four hours. Alternatively, the upright chronic
admittance AU may be obtained after the patient has moved to the
second posture for a different time period, such as one hour, two hours,
five hours, and the like.

[0035] The upright chronic admittance AU may be measured as the
largest impedance vector between the predetermined combination of
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 (shown
in FIG. 1) over a time window. As described above, the IMD 100 (shown in
FIG. 1) may periodically measure the impedance vector between the
predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 throughout the day and night. The IMD 100 may
determine which of the impedance vectors measured during the day (such as
6 a.m. to 6 p.m.) is the largest of the impedance vectors. The impedance
vector obtained during the day may be obtained when the patient is likely
to be upright and corresponding to a period of time when intracardiac and
intrathoracic fluid volumes have reached a minimum state during the day.
The IMD 100 may then calculate the upright chronic admittance AU
from the impedance vector using Equation 1 above. In another embodiment,
the upright chronic admittance AU may be based on two or more
impedance vectors and/or on one or more impedance vectors that are not
the largest impedance vector measured over a time period. By way of
example only, the upright chronic admittance AU may be calculated as
one or more of a mean, median, deviation, and the like, of several
impedance vectors obtained when the patient is likely to be upright.

[0036] At 406, a supine acute admittance (aS) is measured between the
predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1) after the patient transitions
to the first posture. The supine acute admittance aS may be obtained
in an in-clinic setting, such as a physician's office or hospital, by
measuring the impedance vector between the predetermined combination of
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 shortly
after the patient has moved to the first posture. By way of example only,
the supine acute admittance aS may be measured within a sufficiently
short time period after the patient transitions from an upright standing
posture to a supine posture such that fluids within the various fluid
compartments have not have had a chance to equilibrate and the fluid
volume within the slower responding interstitial space has not reached a
steady state. However, a sufficient amount of time has elapsed to acutely
alter the interelectrode spacing and to permit the fast responding
intravascular fluid volume to reach a new steady state. For example, the
supine acute admittance aS may be measured after the patient lies
down and before the fluid in the various chambers of the heart 102 (shown
in FIG. 1) and other thoracic chambers reaches equilibrium.

[0037] The supine acute admittance aS may be measured by a physician
using the IMD 100 (shown in FIG. 1). The physician may use an external
device 558 (shown in FIG. 5) to direct the IMD 100 to obtain the supine
acute admittance aS shortly after the patient has moved to the first
posture, such as within a predetermined time window after the patient has
moved to the first posture. The supine acute admittance aS may be
based on the smallest impedance vector measured shortly after the patient
has moved to the first posture which corresponds to a state when
intravascular fluid volume may have reached a new maximum over a
predetermined time period following the change in posture. Alternatively,
the supine acute admittance aS may be based on two or more impedance
vectors and/or on an impedance vector that is not the smallest impedance
vector measured within a time window after the patient moves to the first
posture. In one embodiment, the supine acute admittance aS may be
measured by measuring the impedance vector between the predetermined
combination of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 within one minute after the patient have moved to the first
posture. Alternatively, the supine acute admittance aS may be
obtained within a different time period after the patient has moved to
the first posture, such as within 40 seconds, 30 minutes, one hour, two
hours, and the like. In another embodiment, the supine acute admittance
aS may be calculated as one or more of a mean, median, deviation,
and the like, of several impedance vectors obtained when the patient is
in a supine position.

[0038] At 408, an upright acute admittance (aU) is measured between
the predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1) after the patient moves to the
second posture. Similar to the supine acute admittance aS, the
upright acute admittance aU may be obtained in an in-clinic setting
by measuring the impedance vector between the predetermined combination
of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134
shortly after the patient has moved to the second posture, such as within
a predetermined time period of moving to the second posture. By way of
example only, the upright acute admittance aU may be measured within
a sufficiently short time period after the patient moves from a supine
posture to an upright posture such that fluids within the various fluid
compartments have not have had a chance to equilibrate and the fluid
volume within the slower responding interstitial space has not reached a
steady state. However, a sufficient amount of time has elapsed to acutely
alter the interelectrode spacing and to permit the fast responding
intravascular fluid volume to reach a new steady state. For example, the
upright acute admittance aU may be measured after the patient stands
up from a supine position and before the fluid in the various chambers of
the heart 102 (shown in FIG. 1) and other thoracic chambers equilibrate.

[0039] The upright acute admittance aU may be measured by a physician
using the IMD 100 (shown in FIG. 1). The physician may use the external
device 558 (shown in FIG. 5) to direct the IMD 100 to obtain the upright
acute admittance aU shortly after the patient has moved to the
second posture. In one embodiment, the upright acute admittance aU
may be based on the largest impedance vector between the predetermined
combination of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 within one minute after the patient have moved to the second
posture which corresponds to a state when intravascular fluid volume may
have reached a new minimum during a predetermined time period following a
change in posture. Alternatively, the upright acute admittance aU
may be obtained within a different time period after the patient has
moved to the second posture, such as within 40 seconds, 30 minutes, one
hour, two hours, and the like. In another embodiment, the upright acute
admittance aU may be based on two or more impedance vectors and/or
an impedance vector that is not the largest impedance vector within the
time window. For example, the upright acute admittance aU may be
calculated as one or more of a mean, median, deviation, and the like, of
several impedance vectors obtained when the patient is upright.

[0040] At 410, the offset factor β is derived for the predetermined
combination of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 (shown in FIG. 1) and for the movement of the patient from the
first posture to the second posture. The offset factor β is based on
the supine chronic and acute admittances (AS and aS) and the
upright chronic and acute admittances (AU and aU). For example,
the offset factor β may be based on chronic and acute changes in
impedance vectors that are measured when the patient moves between
postures.

[0041] In a patient where no offset factor β is needed to correct
impedance vectors obtained from the predetermined combination of
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, the
following relationship may apply between the chronic and acute
admittances AS, AU, aS, aU:

ΔA=C×Δa (Eqn. 2)

where AA represents a difference between the chronic admittances
(AS, AU), C represents an adjustment factor, and Δa
represents a difference between the acute admittances (aS, aU).
In one embodiment, the relationship shown in Equation 2 may be
represented as follows:

AS-AU=C×(aS-aU) (Eqn. 3)

[0042] In one embodiment, the adjustment factor C has a value of 4 which
represents the relative ratio between the fluid volume distributed in
both the intravascular and interstitial fluid compartments and the fluid
volume distributed in the intravascular fluid compartment alone.
Alternatively, the adjustment factor C may have a different value, such
as a value between 3 and 5. The adjustment factor C may be similar to the
adjustment factor described in U.S. Patent Application Publication No.
2008/0262361, entitled "System and Method for Calibrating Cardiac
Pressure Measurements Derived From Signals Detected by an Implantable
Medical Device."

[0043] The left side of Equation 3 represents the change between the
measured chronic supine and upright admittances after a sufficient amount
of time has allowed the various fluid compartments to equilibrate
following the posture change, while the right side of Equation 3
represents the change between the measured acute supine and upright
admittances multiplied by C after a sufficient amount of time has allowed
only the intravascular fluid compartment to reach a new steady state. It
is assumed here that the measured admittances are proportional to the
corresponding fluid volumes within the various compartments. The factor C
may be defined to represent the relative fluid volume ratio between the
combined intravascular and interstitial fluid compartments and the
intravascular fluid compartment alone.

[0044] Using the relationship between the admittances AS, AU,
aS, aU and the impedance vectors shown above in Equation 1,
Equation 3 may be expressed as follows:

where ZS is the impedance vector that corresponds to the supine
chronic admittance AS; ZU is the impedance vector that
corresponds to the upright chronic admittance AU; ζS is
the impedance vector that corresponds to the supine acute admittance
aS; and ζU is the impedance vector that corresponds to the
upright acute admittance aU.

[0045] In a patient where the offset factor β is needed to correct
impedance vectors measured by the IMD 100 (shown in FIG. 1), however, the
offset factor β is included in the relationship between the
impedance vectors that are associated with the chronic and acute
admittances AS, AU, aS, aU set forth above in
Equation 4. For example, the offset factor β adjusts impedance
vectors that are affected by the patient moving to the second posture,
such as an upright position. In one embodiment, the relationship shown
above in Equation 4 is changed to reduce the impedance vectors obtained
when the patient is in the second posture, or an upright position, by the
offset factor β:

[0046] A quadratic equation solution is used to solve for the potential
values of the offset factor β appearing in Equation 5. In one
embodiment, the potential values of the offset factor β may be
represented by the following relationship:

In Equations 7 through 9, ΔZ represents a difference between
ZU and ZS and Δζ represents a difference between
ζU and ζS. The values for the offset factor β
may be expressed in terms of ohms. Two values may be determined from the
quadratic equation solution shown above in Equations 6 through 9.

[0047] At 412, one of the two values for the offset factor β is used
to adjust admittance measurements or impedance vectors obtained between
the predetermined combination of electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1) when the patient moves to the
second posture during a change in position of the patient or during
patient activity. In one embodiment, the lower of the two values that are
calculated from Equation 5 is used for the offset factor β.
Alternatively, the larger of the two values may be used. For example, if
the offset factor β is derived from impedance vectors 138 (shown in
FIG. 1) between the LV ring electrode 134 (shown in FIG. 1) and the
housing 104 when the patient moves from a first supine posterior posture
to a second upright posture, then the offset factor β may be added
to future impedance vectors 138 measured between the LV ring electrode
134 and the housing 104 when the patient moves from a supine posture to
an upright standing posture. As described above, different offset factors
β may be derived for different electrode 104, 116, 118, 120, 122,
124, 126, 128, 130, 132, 134 combinations and/or different changes in
posture.

[0048] Table 1 shown below includes several offset factors β that are
derived to adjust impedance vectors obtained between several different
combinations of electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130,
132, 134 (shown in FIG. 1) when the patient moves from a supine posture
to an upright standing posture. Different tables of the offset factor
β may be derived for different changes in posture by the patient.
For example, a table may include the offset factors β that are
applied to impedance vectors when the patient moves from a supine posture
to an upright standing posture.

[0049] By way of example only, Table 1 shows that the offset factor
β1 may be subtracted from the impedance vectors obtained using
the "A" combination of electrodes 104, 134 (shown in FIG. 1) when the
patient transitions from the supine posture to the upright standing
posture. The offset factor β2 is added to impedance vectors
obtained using the "B" combination of electrodes 130, 104, the offset
factor β3 is added to impedance vectors measured using the "C"
combination of electrodes 104, 132 (shown in FIG. 1), and the offset
factor β4 is added to impedance vectors measured using the "D"
combination of electrodes 120, 126 (shown in FIG. 1) when the patient
transitions from the supine posture to the upright standing posture or
when the patient's activity results in changing postures from the supine
posture to the upright standing posture.

[0051] The IMD 100 includes a programmable microcontroller 522, which
controls the operation of the IMD 100. The microcontroller 522 (also
referred to herein as a processor, processor module, or unit) typically
includes a microprocessor, or equivalent control circuitry, and may be
specifically designed for controlling the delivery of stimulation therapy
and may further include RAM or ROM memory, logic and timing circuitry,
state machine circuitry, and I/O circuitry. The microcontroller 522 may
include one or more modules and processors configured to perform one or
more of the operations described above in connection with the method 400
(shown in FIG. 4).

[0052] An impedance measurement module 524 obtains impedance vectors
between predetermined combinations of the electrodes 104, 116, 118, 120,
122, 124, 126, 128, 130, 132, 134 (shown in FIG. 1). The impedance
measurement module 524 communicates with an impedance measurement circuit
526 by way of a control signal 528 to control which of the electrodes
104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 are used to obtain
an impedance vector. The impedance measuring circuit 526 may be
electrically coupled to a switch 538 so that an impedance vector between
any desired combination of the electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 may be obtained.

[0053] A timing module 530 associates sampling times with impedance
vectors. A sampling time is a time of the day, such as 2 a.m., that is
associated with a time at which the impedance measurement module 524
obtains an impedance vector from a predetermined combination of the
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 (shown
in FIG. 1). The timing module 530 may place or associate the impedance
vectors with time stamps that indicate when each impedance vector was
obtained. The time stamps and impedance vectors may be stored in and
accessible from a tangible and non-transitory computer readable storage
medium, such as a memory 532.

[0054] A correction module 534 adjusts the impedance vectors obtained by
the impedance measuring module 524. As described above, the correction
module 534 may adjust the impedance vectors by the offset factor β
when the patient changes postures. In one embodiment, the correction
module 534 obtains the value of the offset factor β to be applied to
impedance vectors measured between a predetermined combination of the
electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 (shown
in FIG. 1) from the memory 532. Alternatively, the correction module 534
may derive the value or values of the offset factor β based on
previously acquired impedance vectors, as described above. The correction
module 534 communicates with the patient position sensor 136 in order to
determine the postures of the patient. For example, the correction module
534 may communicate with the sensor 136 to determine the previous posture
of a patient and the current posture of the patient in order to determine
which offset factor β to apply to the impedance vectors.

[0056] The switch 538 includes a plurality of switches for connecting the
desired electrodes 104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134
(shown in FIG. 1) and input terminals 500, 502, 504, 506, 508, 510, 512,
514, 516, 518, 520 to the appropriate I/O circuits. The switch 538 closes
and opens switches to provide electrically conductive paths between the
circuitry of the IMD 100 and the input terminals 500, 502, 504, 506, 508,
510, 512, 514, 516, 518, 520 in response to a control signal 540. An
atrial sensing circuit 542 and a ventricular sensing circuit 544 may be
selectively coupled to the leads 110, 112, 114 (shown in FIG. 1) of the
IMD 100 through the switch 538 for detecting the presence of cardiac
activity in the chambers of the heart 102 (shown in FIG. 1). The sensing
circuits 542, 544 may sense the cardiac signals that are analyzed by the
microcontroller 522. Control signals 546, 548 from the microcontroller
522 direct output of the sensing circuits 542, 544 that are connected to
the microcontroller 522.

[0057] The IMD 100 additionally includes a battery 550 that provides
operating power to the circuits shown within the housing 104, including
the microcontroller 522. The IMD 100 may include a physiologic sensor 552
that may be used to adjust pacing stimulation rate according to the
exercise state of the patient.

[0058] The memory 532 may be embodied in a tangible computer-readable
storage medium such as a ROM, RAM, flash memory, or other type of memory.
The microcontroller 522 is coupled to the memory 532 by a data/address
bus 554. The memory 532 may store programmable operating parameters used
by the microcontroller 522, as required, in order to customize the
operation of IMD 100 to suit the needs of a particular patient. For
example, the memory 532 may store values of the offset factor β for
impedance vectors obtained using different combinations of the electrodes
104, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134 (shown in FIG. 1)
and/or for the patient switching between different postures. The memory
532 may store impedance vectors and/or admittances measured by the IMD
100 along with the time stamps associated with the vectors and/or
impedances. The operating parameters of the IMD 100 and offset factors
β may be non-invasively programmed into the memory 532 through a
telemetry circuit 556 in communication with an external device 558, such
as a trans-telephonic transceiver or a diagnostic system analyzer. The
telemetry circuit 556 is activated by the microcontroller 522 by a
control signal 560. The telemetry circuit 556 allows data and status
information relating to the operation of IMD 100 to be sent to the
external device 558 through an established communication link 562.

[0059] An atrial pulse generator 564 and a ventricular pulse generator 566
generate pacing stimulation pulses for delivery by the IMD 100 via the
switch bank 538. The pulse generators 564, 566 are controlled by the
microcontroller 522 via appropriate control signals 568, 570
respectively, to trigger or inhibit the stimulation pulses. To provide
the function of an implantable cardioverter/defibrillator (ICD), the
microcontroller 522 may control a shocking circuit 572 by way of a
control signal 574. The shocking pulses are applied to the patient's
heart 102 (shown in FIG. 1) through at least two shocking electrodes,
such as the LA coil electrode 124 (shown in FIG. 1), the RV coil
electrode 130 (shown in FIG. 1), and/or the SVC coil electrode 132 (shown
in FIG. 1).

[0060] FIG. 6 illustrates a functional block diagram of the external
programming device 558, such as a programmer, that is operated by a
physician, a health care worker, or a patient to interface with IMD 100
(shown in FIG. 1). The external device 558 may be utilized in a hospital
setting, a physician's office, or even the patient's home to communicate
with the IMD 100 to change a variety of operational parameters regarding
the therapy provided by the IMD 100 as well as to select among
physiological parameters to be monitored and recorded by the IMD 100. For
example, the external device 558 may be used to program or update offset
factors β stored in the memory 532 (shown in FIG. 5) of the IMD 100
and that are used in conjunction with impedance vectors obtained by
different combinations of the electrodes 104, 116, 118, 120, 122, 124,
126, 128, 130, 132, 134 (shown in FIG. 1). The external device 532 may
receive impedance vectors obtained by the IMD 100 in order to calculate
offset factors (3.

[0061] The external device 558 includes an internal bus 600 that
connects/interfaces with a Central Processing Unit (CPU) 602, ROM 604,
RAM 606, a hard drive 608, a speaker 610, a printer 612, a CD-ROM or DVD
drive 614, a floppy or disk drive 616, a parallel I/O circuit 618, a
serial I/O circuit 620, a display 622, a touch screen 624, a standard
keyboard connection 626, custom keys 628, and a telemetry subsystem 630.
The internal bus 600 is an address/data bus that transfers information
(for example, either memory data or a memory address from which data will
be either stored or retrieved) between the various components described.
The hard drive 608 may store operational programs as well as data, such
as offset factors β and the like.

[0062] The CPU 602 typically includes a microprocessor, a
micro-controller, or equivalent control circuitry, designed specifically
to control interfacing with the external device 558 and with the IMD 100
(shown in FIG. 1). The CPU 602 may further include RAM or ROM memory,
logic and timing circuitry, state machine circuitry, and I/O circuitry to
interface with the IMD 100. Typically, the microcontroller 522 (shown in
FIG. 5) includes the ability to process or monitor input signals (for
example, data) as controlled by program code stored in memory (for
example, ROM 604).

[0063] The display 622 (for example, may be connected to a video display
632) and the touch screen 624 display text, alphanumeric information,
data and graphic information via a series of menu choices to be selected
by the user relating to the IMD 100 (shown in FIG. 1), such as for
example, status information, operating parameters, therapy parameters,
patient status, access settings, software programming version, offset
factors β, impedance vectors, admittances, thresholds, and the like.
The touch screen 624 accepts a user's touch input 634 when selections are
made. The keyboard 626 (for example, a typewriter keyboard 636) allows
the user to enter data to the displayed fields, operational parameters,
therapy parameters, as well as interface with the telemetry subsystem
630. Furthermore, custom keys 628 turn on/off 638 (for example, EVVI) the
external device 558. The printer 612 prints hard-copies of reports 640
for a physician/healthcare worker to review or to be placed in a patient
file, and speaker 610 provides an audible warning (for example, sounds
and tones 642) to the user in the event a patient has any abnormal
physiological condition occur while the external device 558 is being
used. The parallel I/O circuit 618 interfaces with a parallel port 644.
The serial I/O circuit 620 interfaces with a serial port 646. The drive
616 accepts disks or diskettes 648. The drive 614 accepts CD and/or DVD
ROMs 650.

[0064] The telemetry subsystem 630 includes a central processing unit
(CPU) 652 in electrical communication with a telemetry circuit 654, which
communicates with both an ECG circuit 656 and an analog out circuit 658.
The ECG circuit 656 is connected to ECG leads 660. The telemetry circuit
654 is connected to a telemetry wand 662. The analog out circuit 630
includes communication circuits, such as a transmitting antenna,
modulation and demodulation stages (not shown), as well as transmitting
and receiving stages (not shown) to communicate with analog outputs 664.
The external device 558 may wirelessly communicate with the IMD 100
(shown in FIG. 1) and utilize protocols, such as Bluetooth, GSM, infrared
wireless LANs, HIPERLAN, 3G, satellite, as well as circuit and packet
data protocols, and the like. A wireless RF link utilizes a carrier
signal that is selected to be safe for physiologic transmission through a
human being and is below the frequencies associated with wireless radio
frequency transmission. Alternatively, a hard-wired connection may be
used to connect the external device 558 to the IMD 100 (for example, an
electrical cable having a USB connection).

[0065] FIG. 7 illustrates a distributed processing system 700 in
accordance with one embodiment. The distributed processing system 700
includes a server 702 that is connected to a database 704, a programmer
706 that may similar to the external device 558 described above and shown
in FIG. 5), a local RF transceiver 708, and a user workstation 710
electrically connected to a communication system 712. The communication
system 712 may be an internet, the Internet or a portion thereof, a voice
over IP (VoIP) gateway, a local plain old telephone service (POTS), such
as a public switched telephone network (PSTN), and the like.
Alternatively, the communication system 712 may be a local area network
(LAN), a campus area network (CAN), a metropolitan area network (MAN), or
a wide area network (WAM). The communication system 712 serves to provide
a network that facilitates the transfer/receipt of cardiac signals,
processed cardiac signals, histograms, trend analysis and patient status,
and the like.

[0066] The server 702 is a computer system that provides services to other
computing systems (for example, clients) over a computer network. The
server 702 acts to control the transmission and reception of information
such as cardiac signals, offset factors β, impedance vectors,
admittances, statistical analysis, trend lines, and the like. The server
702 interfaces with the communication system 712, such as the internet,
Internet, or a local POTS based telephone system, to transfer information
between the programmer 706, the local RF transceiver 708, the user
workstation 710 (as well as other components and devices) to the database
704 for storage/retrieval of records of information. By way of example
only, these other components and devices may include a cell phone 714
and/or a personal data assistant (PDA) 716. The server 702 may download,
via a wireless connection 720, to the cell phone 714 or the PDA 716 the
results of processed cardiac signals, offset factors β, postures,
impedance vectors, admittances, or a patient's physiological state based
on previously recorded cardiac information, impedance vectors, postures,
and the like. The server 702 may upload raw cardiac signals (for example,
unprocessed cardiac data) from a surface ECG unit 722 or an IMD 724, such
as the IMD 100 (shown in FIG. 1), via the local RF transceiver 708 or the
programmer 706.

[0067] Database 704 is any commercially available database that stores
information in a record format in electronic memory. The database 704
stores information such as raw cardiac data, processed cardiac signals,
offset factors β, impedance vectors and/or admittances with
associated time stamps, postures, statistical calculations (for example,
averages, modes, standard deviations), histograms, and the like. The
information is downloaded into the database 704 via the server 702 or,
alternatively, the information is uploaded to the server 702 from the
database 704.

[0068] The programmer 706 may be similar to the external device 558 shown
in FIG. 5 and described above, and may reside in a patient's home, a
hospital, or a physician's office. The programmer 706 interfaces with the
surface ECG unit 722 and the IMD 724. The programmer 706 may wirelessly
communicate with the IMD 724 and utilize protocols, such as Bluetooth,
GSM, infrared wireless LANs, HIPERLAN, 3G, satellite, as well as circuit
and packet data protocols, and the like. Alternatively, a hard-wired
connection may be used to connect the programmer 706 to IMD 724 (for
example, an electrical cable having a USB connection). The programmer 706
is able to acquire cardiac signals from the surface of a person (for
example, ECGs), or the programmer 706 is able to acquire intra-cardiac
electrogram (for example, IEGM) signals from the IMD 724. The programmer
706 interfaces with the communication system 712, either via the
internet, Internet, and/or via POTS, to upload the data acquired from the
surface ECG unit 722 or the IMD 724 to the server 702.

[0069] The local RF transceiver 708 interfaces with the communication
system 712 to upload data acquired from the surface ECG unit 722 or the
IMD 724 to the server 702. In one embodiment, the surface ECG unit 722
and the IMD 724 have a bi-directional connection with the local RF
transceiver 708 and/or programmer 706 via a wireless connection 726, 728.
The local RF transceiver 708 is able to acquire cardiac signals from the
surface of a person (for example, ECGs), or acquire data from the IMD
724. On the other hand, the local RF transceiver 708 may download stored
data from the database 704 or the IMD 724.

[0070] The user workstation 710 may interface with the communication
system 712 to download data via the server 702 from the database 704.
Alternatively, the user workstation 710 may download raw data from the
surface ECG unit 722 or IMD 724 via either the programmer 706 or the
local RF transceiver 708. Once the user workstation 710 has downloaded
the data (for example, raw cardiac signals, impedance vectors and/or
admittances with associated time stamps, offset factors β, postures,
and the like), the user workstation 710 may process the data. For
example, the user workstation 710 may be used to calculate various offset
factors β for different combinations of electrodes and/or posture
changes, as described above. Once the user workstation 710 has finished
performing its calculations, the user workstation 710 may either download
the results to the IMD 724 via the local RF transceiver 708 and/or
programmer 706, the cell phone 714, the PDA 716, or to the server 702 to
be stored on the database 704.

[0071] FIG. 8 illustrates a block diagram of exemplary manners in which
embodiments of the present invention may be stored, distributed and
installed on a tangible and non-transitory computer-readable medium. In
FIG. 8, the "application" represents one or more of the methods and
process operations discussed above. For example, the application may
represent the processes carried out in connection with FIGS. 1 through 7
as discussed above.

[0072] As shown in FIG. 8, the application is initially generated and
stored as source code 800 on a tangible and non-transitory source
computer-readable medium 802. The source code 800 is then conveyed over
path 804 and processed by a compiler 806 to produce object code 808. The
object code 808 is conveyed over path 810 and saved as one or more
application masters on a tangible and non-transitory master
computer-readable medium 812. The object code 808 may then be copied
numerous times, as denoted by path 814, to produce production application
copies 816 that are saved on separate tangible and non-transitory
production computer-readable media 818. The production computer-readable
media 818 are then conveyed, as denoted by path 820, to various systems,
devices, terminals and the like. In the example of FIG. 8, a user
terminal 822, a device 824, and a system 826 are shown as examples of
hardware components, on which the production computer-readable media 818
are installed as applications (as denoted by 828, 830, 832). For example,
the production computer-readable media 818 may be installed on one or
more of the IMD 100 (shown in FIG. 1), the user workstation 710 (shown in
FIG. 7), the server 702 (shown in FIG. 7), the database 704 (shown in
FIG. 7), the cell phone 714 (shown in FIG. 7), the PDA 716 (shown in FIG.
7), the programmer 706 (shown in FIG. 7), and the like.

[0073] The source code 800 may be written as scripts, or in any high-level
or low-level language. Examples of the source, master, and production
computer-readable medium 802, 812, and 818 include, but are not limited
to, tangible media such as CD-ROM, DVD-ROM, RAM, ROM, flash memory, RAID
drives, memory on a computer system and the like. Examples of the paths
804, 810, 814, 820 include, but are not limited to, network paths, the
internet, Bluetooth, GSM, infrared wireless LANs, HIPERLAN, 3G,
satellite, and the like. The paths 804, 810, 814, 820 may also represent
public or private carrier services that transport one or more physical
copies of the source, master, or production computer-readable media 802,
812, 816 between two geographic locations. The paths 804, 810, 814, 820
may represent threads carried out by one or more processors in parallel.
For example, one computer may hold the source code 800, compiler 806, and
object code 808. Multiple computers may operate in parallel to produce
the production application copies 816. The paths 804, 810, 814, 820 may
be intra-state, inter-state, intra-country, inter-country,
intra-continental, inter-continental and the like.

[0074] The operations noted in FIG. 8 may be performed in a widely
distributed manner world-wide with only a portion thereof being performed
in the United States. For example, the application source code 800 may be
written in the United States and saved on a source computer-readable
medium 802 in the United States, but transported to another country
(corresponding to path 804) before compiling, copying and installation.
Alternatively, the application source code 800 may be written in or
outside of the United States, compiled at a compiler 806 located in the
United States and saved on a master computer-readable medium 812 in the
United States, but the object code 808 transported to another country
(corresponding to path 814) before copying and installation.
Alternatively, the application source code 800 and object code 808 may be
produced in or outside of the United States, but production application
copies 816 produced in or conveyed to the United States (for example, as
part of a staging operation) before the production application copies 816
are installed on user terminals 822, devices 824, and/or systems 826
located in or outside the United States as applications 828, 830, 832.

[0075] As used throughout the specification and claims, the phrases
"computer-readable medium" and "instructions configured to" shall refer
to any one or all of (i) the source computer-readable medium 802 and
source code 800, (ii) the master computer-readable medium and object code
808, (iii) the production computer-readable medium 818 and production
application copies 816 and/or (iv) the applications 828, 830, 832 saved
in memory in the terminal 822, device 824, and system 826.

[0076] In accordance with certain embodiments, methods, systems, and
devices are provided that are able to adjust impedance vectors and/or
admittances based on changes in a patient's posture. The adjustments may
be used to modify the impedance vectors and/or admittances in order to
compensate for posture dependent changes in the interelectrode spacing
and geometry so that physiological parameters such as LAP may be
estimated more accurately.

[0077] It is to be understood that the above description is intended to be
illustrative, and not restrictive. For example, the above-described
embodiments (and/or aspects thereof) may be used in combination with each
other. In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without departing
from its scope. While the dimensions and types of materials described
herein are intended to define the parameters of the invention, they are
by no means limiting and are exemplary embodiments. Many other
embodiments will be apparent to those of skill in the art upon reviewing
the above description. The scope of the invention should, therefore, be
determined with reference to the appended claims, along with the full
scope of equivalents to which such claims are entitled. In the appended
claims, the terms "including" and "in which" are used as the
plain-English equivalents of the respective terms "comprising" and
"wherein." Moreover, in the following claims, the terms "first,"
"second," and "third," etc. are used merely as labels, and are not
intended to impose numerical requirements on their objects. Further, the
limitations of the following claims are not written in
means-plus-function format and are not intended to be interpreted based
on 35 U.S.C. §112, sixth paragraph, unless and until such claim
limitations expressly use the phrase "means for" followed by a statement
of function void of further structure.

[0078] This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in the art
to practice the invention, including making and using any devices or
systems and performing any incorporated methods. The patentable scope of
the invention is defined by the claims, and may include other examples
that occur to those skilled in the art. Such other examples are intended
to be within the scope of the claims if they have structural elements
that do not differ from the literal language of the claims, or if they
include equivalent structural elements with insubstantial differences
from the literal languages of the claims.